The operator of the Fukushima nuclear power plant has retracted an earlier assessment that a continuous nuclear reaction, or a criticality, could have taken place in the damaged Number 2 reactor.

The Tokyo Electric Power Company, or TEPCO, said on Thursday that the small amount of xenon-135 it detected in gas taken from the reactor’s containment vessel was the result of the spontaneous nuclear fission of radioactive curium-242 and -244. The two substances are contained in nuclear fuel.

The amount of xenon-135 detected almost matched the amount that would have been produced if the radioactive curium in the fuel had undergone spontaneous fission.

TEPCO says a criticality event would have resulted in higher levels of xenon concentration.

Spontaneous fission refers to the nuclear fission of radioactive materials other than uranium, and it does not lead to criticality. Such fission is said to occur constantly.

The earlier detection of small amounts of Xenon-135 had suggested the possibility of a criticality occurrence in the melted fuel in the damaged reactor.

TEPCO says it will send the assessment to the Nuclear and Industrial Safety Agency for reevaluation.

Officials of Tokyo Electric Power Co. (TEPCO) pumped water mixed with boric acid into the No. 2 reactor at the Fukushima No. 1 nuclear power plant early on Nov. 2 after finding traces of xenon, a radioactive gas that might indicate nuclear fission has taken place.

The operator of the crippled Fukushima Daiichi nuclear power plant says it found in the facility’s No.2 reactor radioactive substances that could have resulted from continuous nuclear fission.

The Tokyo Electric Power Company, or TEPCO, said on Wednesday that it detected xenon-133 and xenon-135 in gas taken from the reactor’s containment vessel on the previous day. The substances were reportedly in concentrations of 6 to more than 10 parts per million becquerels per cubic centimeter.

Xenon-135 was also detected in gas samples collected on Wednesday.

Radioactive xenon is produced during nuclear fission.
The half-life of xenon-133 is 5 days, and that of xenon-135 is 9 hours.

TEPCO says the findings suggest that nuclear fission may have occurred recently, not just after the March 11th accident, and that a state of criticality could have occurred temporarily in some areas.

TEPCO workers poured a boric acid solution into the reactor on Wednesday to suppress nuclear fission.

The utility says it has not found any significant change in temperature and pressure of the reactor, and that large-scale criticality did not occur.

TEPCO says the reactor’s cooling process is continuing and that the firm expects to achieve cold shutdown at the plant this year as planned. But the utility also says it wants to take a close look at the situation of the plant’s No.1 and 3 reactors.

Officials of Tokyo Electric Power Co. (TEPCO) pumped water mixed with boric acid into the No. 2 reactor at the Fukushima No. 1 nuclear power plant early on Nov. 2 after finding traces of xenon, a radioactive gas that might indicate nuclear fission has taken place.

Officials of the company said that some parts of the reactor may have reached criticality, a state of self-sustaining nuclear fission. Fuel believed to have melted in the accident triggered by the March 11 earthquake may have caused the fission. The boric acid was pumped into the reactor to suppress the reactions.

Radiation levels near the Fukushima plant have not shown any unusual surge, but the latest development may hinder plans by the central government to bring the Fukushima nuclear accident under control by the end of the year.

TEPCO workers began operating a gas control system at the No. 2 reactor from Oct. 28 to purify gases within the containment vessel in an attempt to reduce the leaking of radioactive material.

TEPCO officials said a test on Nov. 1 of the gases processed found traces of what appeared to be radioactive xenon 133 and xenon 135.

The identification of the gas as xenon is now being checked by another research institute. Radioactive xenon is produced when uranium in fuel in a reactor core undergoes nuclear fission.

Xenon 133 has a half-life of about five days while xenon 135 has a half-life of about nine hours.

If the gas found in the reactor was xenon, it would mean nuclear fission is continuing in the reactor.

Because of such results, TEPCO officials mixed boric acid into water pumped into the reactor for cooling purposes from 2:48 a.m. on Nov. 2.

However, no radioactive iodine, which is also produced during nuclear fission with xenon, was detected in the gas checked at the No. 2 reactor.

Moreover, neutron rays, which are emitted during nuclear fission, were not detected in the vicinity of the reactor building.

“We cannot deny the possibility of a temporary, small-scale state of criticality,” said TEPCO official Junichi Matsumoto. “However, because there was no sudden increase in the temperature or pressure of the reactor core, we concluded there was no major state of criticality.”

Large scale criticality will not usually occur unless, as is found in a normal reactor core, nuclear fuel is carefully positioned and surrounded by water to stimulate nuclear fission.

Through a constant pumping of water into the cores of the No. 1 to No. 3 reactors, temperatures at all three reactors had fallen under 100 degrees by late September, six months after the nuclear accident, indicating stable cooling of the reactors.

For that reason, central government and TEPCO officials had thought they were on course to bring the situation at the Fukushima plant under control by the end of the year.

The temperature at the bottom of the pressure vessel of the No. 2 reactor was 76 degrees as of 5 a.m. on Nov. 2.

During the measurement of gases by the gas control system, the concentration of hydrogen rose from 1 percent to 2.7 percent on Oct. 30. Hydrogen is produced when water is broken down by radiation from the nuclear fuel, and it is possible that the increase in hydrogen may have been related to nuclear fission in the reactor.

Additional nitrogen was pumped into the reactor to prevent an explosion of the sort triggered at the Fukushima plant in March.

TEPCO officials said it was the first time xenon had been detected. However, that may be due to the failure to take appropriate measurements. It is possible that nuclear fission has been producing xenon in the No. 2 reactor since the accident and that similar reactions are occurring in the No. 1 and No. 3 reactors, where nuclear fuel has also melted.

Officials of the Nuclear and Industrial Safety Agency (NISA) held a news conference on the morning of Nov. 2.

“While we cannot deny that nuclear fission may have occurred in some places, (the No. 2 reactor) is, overall, in a stable condition,” said Yoshinori Moriyama, NISA deputy director-general for nuclear accident measures

A small-scale criticality incident–in which nuclear fuel achieves a fission chain reaction–may have taken place at the No. 2 reactor of the crippled Fukushima No. 1 nuclear power plant, as small amounts of radioactive xenon were detected, the plant operator, Tokyo Electric Power Co., said Wednesday.

TEPCO injected boric acid into the reactor, as boric acid is effective in suspending nuclear fission reactions by absorbing neutrons.

There have been no significant temperature fluctuations in the reactor and radiation levels in and around the unit have not shown major changes, according to TEPCO.

The substances were found Tuesday in gas from the reactor containment vessel, Junichi Matsumoto, acting director of TEPCO’s Nuclear Power and Plant Siting Division, said at a press conference.

Xenon 133 has a half-life of about five days, while xenon 135’s is about nine hours. Because the two substances have very short half-lives, a small-scale fission reaction is likely to have taken place within the reactor, he said.

“As the reactor’s cooling is progressing, this finding will not have a major impact on the situation,” Matsumoto said.

However, TEPCO and the government are likely to be forced to make careful decisions as they try to bring the reactors to a stable state of cold shutdown within the year, observers said.

“We don’t believe criticality has been maintained,” Matsumoto said.

TEPCO suspects a nuclear fission chain reaction took place caused by existing neutrons within the reactor or that very local criticality took place as the water temperature in the reactor decreased and water density increased, due to the recent increase of water injection.

At the No. 2 reactor, TEPCO activated in late October a gas management system to remove radioactive substances contained in the gas within the containment vessel with filters. The two types of xenon were detected in the gas.

The amounts detected were about one-100,000th becquerel per cubic centimeter for both types.

Since the amounts of the two substances are extremely small, TEPCO plans to ask the Japan Atomic Energy Agency for another round of checks.

Cooling of the No. 2 reactor had been delayed among the Nos. 1-3 reactors at the Fukushima No. 1 plant. The amount of water injected was increased on Sept. 14.

A nuclear energy expert says the presence of xenon in the No.2 reactor leaves open the possibility that localized and temporary fission could still occur.
Professor Koji Okamoto of the University of Tokyo Graduate School says substances from melted fuel that could undergo fission are probably scattered around, but are unlikely to react.

He says, however, that neutrons from radioactive materials could react with the uranium fuel and other substances.

Okamoto says a self-sustaining chain reaction that creates criticality is unlikely to happen because huge amounts of boric acid have been poured into the reactor.

He adds that these neutrons must be closely monitored to make sure fission does not take place.

TOKYO (Kyodo) — A high level of radiation detected near a supermarket in Tokyo appears to be unrelated to the nuclear crisis at the Fukushima Daiichi power plant, science ministry officials said Tuesday.

An investigation by the ministry detected a high radiation level of around 40 millisieverts per hour near a bottle found 40 centimeters deep in the ground near the supermarket in Setagaya Ward.

If a person were continually exposed to such a level of radiation for two and a half hours, the risk of dying from cancer would increase by 0.5 percent, the officials said.

Although the source of the radiation has not been determined, lead and bismuth, released when radium-226 decays, were detected after workers dug into the ground, they said.

Radium is not among the radioactive substances released by the Fukushima plant since it was crippled by the March 11 earthquake and tsunami, the Ministry of Education, Culture, Sports, Science and Technology said earlier.

The radiation level on the surface of the ground nearby was around 1 millisievert per hour, the officials said. The exposure limit set for ordinary people is 1 millisievert per year.

A resident reported high radiation readings in the area on Friday, prompting the ministry to take measurements, with up to 170 microsieverts per hour recorded at one spot on the surface of a pavement near the supermarket and up to 110 microsieverts per hour recorded at another spot near the shop’s entrance.

No alarming radiation levels were measured elsewhere in the area.

High levels of radiation were also detected along a sidewalk in Setagaya Ward in October. Several dozen bottles containing radium were later found under the floor of a house nearby.

not to menation,Cs-134,137 were measured,but Iodine131 was also measured from various rice samples.

Since this is a private institution,their measurement result is more honest then government’s.

Most of the samples were taken from 10/1~10/20/2011.

This may prove Fukushima went back to recriticality state in September.

Even though Iodine131 remains longer than 7 days,considering it is measured from sewage sludge,soil,in Tokyo,Iwate and Saitama,it may be rational to think Fukushima goes to recriticality state on and off.

High concentrations of radioactive tellurium and silver have been found in soil around the Fukushima No. 1 nuclear power plant, according to the science ministry’s contamination maps released on Oct. 31.

Japan’s nuclear watchdog says it believes another meltdown at the Fukushima Daiichi nuclear power plant is unlikely, but will continue to monitor the situation closely.

The Nuclear and Industrial Safety Agency held a news conference on Wednesday, after radioactive xenon was detected in the containment vessel of the No.2 reactor. The presence of the substance indicates that nuclear fission may have briefly resumed.

The safety agency says it is unlikely that nuclear fuel has begun melting again, as the density of xenon is low and there has been no change in the reactor’s temperatures.

The agency also says it will closely monitor if xenon continues to be detected after Tokyo Electric Power Company poured boric acid solution into the reactor to suppress nuclear fission.

It added that it cannot yet say how the latest development will affect the government’s plan to achieve a cold shutdown of the disabled reactors by the end of the year.

Senior official Yoshinori Moriyama said it is difficult to determine the amount of nuclear fuel remaining in the containment vessel of the No.2 reactor. He said his agency will assess the situation and judge if it is still possible to stably lower temperatures in the reactor and containment vessel.

Another radioactive hot spot in Tokyo’s Setagaya Ward was likely caused by radium and not related to the accident at the Fukushima No. 1 nuclear power plant, officials said.

Officials of the Ministry of Education, Culture, Sports, Science and Technology said radium-226 buried in a parking lot was probably behind the high radiation levels in two locations near a supermarket in the Hachimanyama district of the ward.

In October, officials detected high radiation levels in Tsurumaki, about 3 kilometers from the latest site. Radium was found under the floor of a private home next to where the radiation was detected.

On Nov. 1, one of the locations near the Hachimanyama supermarket was dug up under the watch of science ministry officials. The location had radiation levels of 110 microsieverts per hour.

According to ministry officials, bismuth and lead–elements that are produced when radium decays–were detected after about 30 centimeters of soil was dug up.

A bottle containing a reagent was found after the digging reached 40 centimeters. While the contents were unknown, the surface had radiation levels of about 40 millisieverts per hour. The radiation reading at ground level was 1 millisievert per hour.

Science ministry officials said that simply passing through the area where the reagent was found would pose no health risks.

The land around the supermarket is owned by the Central Union of Agricultural Co-operatives (JA-Zenchu). According to organization officials, a school dorm was constructed on the site in 1929. A school building was later built and the structures rented out after the school closed in 1973.

The site was used to house agricultural interns from abroad as well as a parking lot. The supermarket began using the site in 1999.

According to neighborhood residents, the abandoned lot was covered with weeds about 1 meter high for about a decade after the school closed. Rubbish was often thrown onto the lot.

Science ministry officials plan to dig up the other site where a radiation level of 170 microsieverts per hour was detected. They said it would take one to two weeks to remove the suspect elements from the two sites.

Radium-226, which has a half-life of about 1,600 years, was used in the past for cancer treatment as well as in self-luminous paint.

There have been previous cases of radium found in homes or warehouses, underlining the lax control measures used in the past.

The radium found in the Tsurumaki home in October appears to have been for industrial use. Radiation levels near the area where the radium was found were high, at about 600 microsieverts per hour.

Officials said authorities receive about three reports of such discoveries of radium a year. But the recent interest in testing for radioactive materials following the Fukushima nuclear accident will likely lead to even more reports.

Japanese officials said Saturday that they have detected elevated levels of radiation in a residential area of Chiba.

The hotspot was found on a plot of vacant land in Kashiwa on Friday, after a concerned resident reported high radiation levels, NTV reported. Authorities tested the land and reported a reading of 57.5 microsieverts an hour.

Kashiwa is located in northern Chiba, and has a estimated population of 404,820 people.

The reading is higher than several areas within the 20-kilometer exclusion zone around the crippled Fukushima nuclear plant that was evacuated by the government, according to Science Ministry data.

Authorities said that once they buried the hotspot under sand and a sheet, the reading fell to 0.4 microsieverts an hour, NTV reported.

Despite initial fears that the spike in radiation readings was a result of fallout from the Fukushima nuclear disaster, authorities said there is a low probability that the two are connected.